This invention relates to an improved sensor array for measuring forces. More particularly, the present invention relates to an improved force pin, sensor array for measuring triaxial forces, such as those generated in the footprint of a tire.
The prior art includes a variety of sensors incorporating strain gauges built into specialized instruments designed to measure forces on various objects. For example, U.S. Pat. No. 2,918,816 discloses an improved six-component strain gauge balance system for use in high pressure wind tunnels to measure simultaneously the six major forces and moments corresponding to the six degrees of freedom of any three dimensional body under test. The patent describes a cylindrical sleeve for attaching to the tested object and a core within the sleeve for attaching to a support in the wind tunnel. Annular torsion load cells, axial rhombic-shaped load cells, and diametrical cantilevered load cells are connected between the sleeve and core to measure the various forces.
In relation to pneumatic tires, European Patent No 0 656 269 A1 discloses an essentially two-dimensional array of sensors used to indicate tire inflation by determining the distribution of contact forces over the footprint of a pneumatic tire. The sensors in the array measure only normal force, and are preferably flat detectors such as piezoelectric or piezoresistive polymer film sensors. A related U.S. Pat. No. 5,396,817 concerns similar measurements utilizing a mainly linear array of strain gage sensors.
An example of measuring tire forces is seen in U.S. Pat. No. 4,986,118 (""118) which discloses an array of force sensors, each separately constructed of a vertical hollow tubular member, either square or cylindrical in cross section, with strain gages secured to the vertical surfaces of the member to measure forces applied by a tire to the top bearing surface of the tubular member. Concentrically inside each tubular member is a motion sensor to measure relative tire tread motion. The motion sensor includes an elongated pin having a pointed tip extending beyond the bearing surface of the tubular member so that the tip penetrates the tread of the tire under test, and includes strain gages secured to the pin to indicate the motion of the pin and therefore the motion of the tread of the tire penetrated by the pin.
As disclosed in the ""118 patent, the prior art includes sensors for measuring the contact pressure of a tire. For example, one prior art system included a plurality of individual pressure sensors in a transducer system to measure the local triaxial contact pressure and the tangential slip pressures in the contact patch, i.e. the xe2x80x9cfootprintxe2x80x9d, of a tire. This system allowed for measurements along each rib in a tread pattern of a tire to determine factors such as high local pressures and high slip pressures that cause uneven tire wear. The prior art sensors were stable, temperature compensated, high frequency transducers which were typically mounted in an array that was strong enough to support a moving tire.
The prior art pressure sensors, as described in a product description entitled xe2x80x9cTire-Road Contact Pressure Sensorsxe2x80x9d from PRECISION MEASUREMENT CO. of An Arbor, Mich., included individual cantilever pins electrically connected to a temperature compensated strain gauge system that enabled each of the individual pins to simultaneously measure the vertical contact force, the fore-aft tangential force, and the lateral tangential force. A concern relating to the prior art contact pressure sensors was the use of a pressure member diaphragm (membrane) at the contact surface that was less sensitive at the edges than in the center of the membrane. Also, each of the prior art contact sensor pins was individually mounted which, due to geometric size limitations, enabled a limited number of pins to be joined together for individually measuring the forces generated in the footprint of a tire. In some cases, only a single triaxial force pin sensor was typically used to fully map the forces and pressures generated in the footprint. To fully map the forces generated in a tire footprint with the prior art system, the tire would be passed across a limited number of pressure contact sensors multiple times due to geometric considerations. The geometric consideration mentioned above relates to the minimum center to center distance between adjacent contact pressure sensors.
It is an object of the present invention to provide a triaxial force pin sensor array, the triaxial force pin sensor array being as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
It is an object of the present invention to provide a triaxial force pin sensor array for measuring triaxial forces, such as those generated in the footprint of a tire, that obviates the problems and limitations of the prior art systems.
It is another object of the present invention to provide a triaxial force pin sensor array that incorporates a modular design that allows for rapid replacement of defective sensor array elements.
Another object of the invention is to provide a method of measuring the actual forces generated in the footprint of a tire with a triaxial force pin sensor array that substantially prevents dirt and contamination from affecting force measurements.
Yet another object of the invention is to provide a triaxial force pin sensor array which reduces electromagnetic interference (EMI) and radio frequency (RF) contamination of the sampled data signals.
A still further object of the present invention is to provide a triaxial force pin sensor array which exhibits uniform response in the normal direction, Fz, regardless of contact location with individual triaxial force pins of the sensor array.
Another object of the present invention is to provide a triaxial force pin sensor array that features mechanical overload protection.
Still another object of the invention is to provide a triaxial force pin sensor array which includes a high density of individual force pin sensors.
Accordingly, there is provided a triaxial force pin sensor array module that has two triaxial force pin sensor arrays mounted together. Each of the triaxial force pin sensor arrays has a plurality of cantilever force pins to measure the local normal pressure and the tangential force applied to an upper contact surface of the pins. The individual cantilever force pins have a region of reduced cross section extending around the circumference thereof and two slots disposed in opposite facing directions between the region of reduced cross section and the contact surface. The slots are disposed in spaced relation to the upper contact surface forming a shear plate or web whose sensitivity to the contact pressure is determined by the thickness of the web between the reduced section and the diameter of internal bore extending through the web. Sensors are mounted to the opposite facing side surfaces of the cantilever force pins in the region of reduced cross section and to a surface in between the two slots.
According to the invention, the triaxial force pin sensor array module is preferably constructed of a material having a material proportional limit of at least about 30,000 pounds per square inch (psi) (2.07xc3x97108 newtons/meter2 (n/m2)) and up to about 100,000 psi (6.89xc3x97108 n/m2), such as for example aircraft aluminum or beryllium copper. The high material proportional limit insures that the force pins will move from the force applied thereto without any plastic deformation in the range possible due to the location of an adjacent pin or side wall of the sensor array. The construction material preferably has natural shielding capability which reduces electromagnetic interference (EMI) and radio frequency (RF) contamination of the sampled data.
Further in accordance with the invention, a sealant, such as a silicone rubber, having a modulus between about 100 pounds per square inch (psi) (6.89xc3x97105 n/m2) and about 1000 psi (6.89xc3x97106 n/m2) is disposed about each of the cantilever force pins to prevent dust and dirt particles from accumulating therebetween while not materially affecting the movement of the pins.